After five years I am adding a response... I hope someone still find it useful. A simple way to solve this is to do add a RotateTransform by 180 degrees.
Cheers
<cap:CapPlayer x:Name="player" RenderTransformOrigin="0.5, 0.5">
<cap:CapPlayer.RenderTransform>
<RotateTransform Angle="180"/>
</cap:CapPlayer.RenderTransform>
</cap:CapPlayer>
I guess there might be a way to correct it from the video receive code, but it would take longer to find the code to change, if anyone has the solution please post it.Cheers